session 7 factor analysis

Session 7

Factor Analysis

Chapter Outline

1) Overview

2) Basic Concept

3) Factor Analysis Model

4) Statistics Associated with Factor Analysis

Factor Analysis• Factor analysis is a general name denoting a class of procedures

primarily used for data reduction and summarization. • Factor analysis is an interdependence technique in that an entire

set of interdependent relationships is examined without making the distinction between dependent and independent variables.

• Factor analysis is used in the following circumstances: – To identify underlying dimensions, or factors, that explain the

correlations among a set of variables. – To identify a new, smaller, set of uncorrelated variables to

replace the original set of correlated variables in subsequent multivariate analysis (regression or discriminant analysis).

Factor Analysis ModelThe unique factors are uncorrelated with each other and with the common factors. The common factors themselves can be expressed as linear combinations of the observed variables.

Fi = Wi1X1 + Wi2X2 + Wi3X3 + . . . + WikXk

Where:

Fi = estimate of i th factor

Wi = weight or factor score coefficient

k = number of variables

Conducting Factor Analysis: Formulate the Problem

• The objectives of factor analysis should be identified.

• The variables to be included in the factor analysis should be specified based on past research, theory, and judgment of the researcher. It is important that the variables be appropriately measured on an interval or ratio scale.

• An appropriate sample size should be used. As a rough guideline, there should be at least four or five times as many observations (sample size) as there are variables.

Correlation Matrix

Conducting Factor Analysis:Construct the Correlation Matrix• The analytical process is based on a matrix of correlations

between the variables.

• Bartlett's test of sphericity can be used to test the null hypothesis that the variables are uncorrelated in the population: in other words, the population correlation matrix is an identity matrix. If this hypothesis cannot be rejected, then the appropriateness of factor analysis should be questioned.

• Another useful statistic is the Kaiser-Meyer-Olkin (KMO) measure of sampling adequacy. Small values of the KMO statistic indicate that the correlations between pairs of variables cannot be explained by other variables and that factor analysis may not be appropriate.

Conducting Factor Analysis:Determine the Method of Factor Analysis

• In principal components analysis, the total variance in the data is considered. The diagonal of the correlation matrix consists of unities, and full variance is brought into the factor matrix. Principal components analysis is recommended when the primary concern is to determine the minimum number of factors that will account for maximum variance in the data for use in subsequent multivariate analysis. The factors are called principal components.

• In common factor analysis, the factors are estimated based only on the common variance. Communalities are inserted in the diagonal of the correlation matrix. This method is appropriate when the primary concern is to identify the underlying dimensions and the common variance is of interest. This method is also known as principal axis factoring.

Results of Principal Components Analysis

Communalities

Variables Initial Extraction V1 1.000 0.926 V2 1.000 0.723 V3 1.000 0.894 V4 1.000 0.739 V5 1.000 0.878 V6 1.000 0.790

Initial Eigen values

Factor Eigen value % of variance Cumulat. % 1 2.731 45.520 45.520 2 2.218 36.969 82.488 3 0.442 7.360 89.848 4 0.341 5.688 95.536 5 0.183 3.044 98.580 6 0.085 1.420 100.000

The lower-left triangle contains the reproduced correlation matrix; the diagonal, the communalities; the upper-right triangle, the residuals between the observed correlations and the reproduced correlations.

The lower-left triangle contains the reproduced correlation matrix; the diagonal, the communalities; the upper-right triangle, the residuals between the observed correlations and the reproduced correlations.


Conducting Factor Analysis:Determine the Number of Factors

• A Priori Determination. Sometimes, because of prior knowledge, the researcher knows how many factors to expect and thus can specify the number of factors to be extracted beforehand.

• Determination Based on Eigenvalues. In this approach, only factors

with Eigenvalues greater than 1.0 are retained. An Eigenvalue represents the amount of variance associated with the factor. Hence, only factors with a variance greater than 1.0 are included. Factors with variance less than 1.0 are no better than a single variable, since, due to standardization, each variable has a variance of 1.0. If the number of variables is less than 20, this approach will result in a conservative number of factors.

Conducting Factor Analysis:Determine the Number of Factors

• Determination Based on Scree Plot. A scree plot is a plot of the Eigenvalues against the number of factors in order of extraction. Experimental evidence indicates that the point at which the scree begins denotes the true number of factors. Generally, the number of factors determined by a scree plot will be one or a few more than that determined by the Eigenvalue criterion.

• Determination Based on Percentage of Variance. In this approach the

number of factors extracted is determined so that the cumulative percentage of variance extracted by the factors reaches a satisfactory level. It is recommended that the factors extracted should account for at least 60% of the variance.

Scree Plot

0.5

2 5 4 3 6 Component Number

0.0

2.0

3.0

Eig

enva

lue

1.0

1.5

2.5

1

Conducting Factor Analysis: Rotate Factors

• Although the initial or unrotated factor matrix indicates the relationship between the factors and individual variables, it seldom results in factors that can be interpreted, because the factors are correlated with many variables. Therefore, through rotation, the factor matrix is transformed into a simpler one that is easier to interpret.

• In rotating the factors, we would like each factor to have nonzero, or significant, loadings or coefficients for only some of the variables. Likewise, we would like each variable to have nonzero or significant loadings with only a few factors, if possible with only one.

• The rotation is called orthogonal rotation if the axes are maintained at right angles.

Conducting Factor Analysis: Rotate Factors

• The most commonly used method for rotation is the varimax procedure. This is an orthogonal method of rotation that minimizes the number of variables with high loadings on a factor, thereby enhancing the interpretability of the factors. Orthogonal rotation results in factors that are uncorrelated.

• The rotation is called oblique rotation when the axes are not maintained at right angles, and the factors are correlated. Sometimes, allowing for correlations among factors can simplify the factor pattern matrix. Oblique rotation should be used when factors in the population are likely to be strongly correlated.

Factor Matrix Before and After Rotation

Factors

(a)High LoadingsBefore Rotation

(b)High LoadingsAfter Rotation

FactorsVariables123456

1XXXXX

2

X

XXX

1X

X

X

2

X

X

X

Variables123456

Conducting Factor Analysis: Interpret Factors

• A factor can then be interpreted in terms of the variables that load high on it.

• Another useful aid in interpretation is to plot the variables, using the factor loadings as coordinates. Variables at the end of an axis are those that have high loadings on only that factor, and hence describe the factor.

Factor Loading Plot

1.0

0.5

0.0

-0.5

-1.0

Component 1

Component Plot in Rotated Space

1.0 0.5 0.0 -0.5 -1.0

V1

V3

V6 V2

V5

V4

Component Variable 1 2

V1 0.962 -2.66E-02

V2 -5.72E-02 0.848

V3 0.934 -0.146

V4 -9.83E-02 0.854

V5 -0.933 -8.40E-02

V6 8.337E-02 0.885

Rotated Component Matrix Component 2

Conducting Factor Analysis:Select Surrogate Variables

• By examining the factor matrix, one could select for each factor the variable with the highest loading on that factor. That variable could then be used as a surrogate variable for the associated factor.

• However, the choice is not as easy if two or more variables have similarly high loadings. In such a case, the choice between these variables should be based on theoretical and measurement considerations.

Statistics Associated with Factor Analysis

• Bartlett's test of sphericity. Bartlett's test of sphericity is a test statistic used to examine the hypothesis that the variables are uncorrelated in the population. In other words, the population correlation matrix is an identity matrix; each variable correlates perfectly with itself (r = 1) but has no correlation with the other variables (r = 0).

• Correlation matrix. A correlation matrix is a lower triangle matrix showing the simple correlations, r, between all possible pairs of variables included in the analysis. The diagonal elements, which are all 1, are usually omitted.


• Communality. Communality is the amount of variance a variable shares with all the other variables being considered. This is also the proportion of variance explained by the common factors.

• Eigenvalue. The eigenvalue represents the total variance explained by each factor.

• Factor loadings. Factor loadings are simple correlations between the variables and the factors.

• Factor or Component matrix. A factor matrix contains the factor loadings of all the variables on all the factors extracted.


• Factor scores. Factor scores are composite scores estimated for each respondent on the derived factors.

• Kaiser-Meyer-Olkin (KMO) measure of sampling adequacy. The Kaiser-Meyer-Olkin (KMO) measure of sampling adequacy is an index used to examine the appropriateness of factor analysis. High values (between 0.5 and 1.0) indicate factor analysis is appropriate. Values below 0.5 imply that factor analysis may not be appropriate.

• Percentage of variance. The percentage of the total variance attributed to each factor.

• Scree plot. A scree plot is a plot of the Eigenvalues against the number of factors in order of extraction.